Transcription and Translation

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Transcription and Translation

• The genetic information is stored in the nucleus and utilized within the cytoplasm
• Stored as a nucleic acid sequence
• Utilized as a protein
• This conversion of stored information into a useful form requires two separate processes
• Transcription formation of message RNA from the DNA template
• Similar to the replication process
• Translation of the mRNA nucleotide sequence into a protein primary amino acid sequence

Need for Messenger RNA (mRNA)

• mRNA conveys information from the chromosome(s) to the ribosomes
• From the genetic library to the protein assembly line
• Evidence for RNA being the messenger
• Sites of protein synthesis have high levels of RNA
• Radioactive studies in eucaryotes have shown the RNA synthesized in the nucleus moves to the cytoplasm
• In vitro translation of RNA results in proteins

RNA versus DNA

• RNA and DNA are quite similar but differ in three ways
• Carbohydrates of the nucleotides in RNA are ribose versus deoxyribose in DNA
• RNA contains uracil instead of the thymine found in DNA
• RNA is orginarily single stranded, while DNA is usually double stranded

Three Types of RNA

• Messenger RNA (mRNA) carries information that specifies the sequence of amino acids to the ribosomes
• Ribosomal RNA (rRNA) forms part of the ribosomes which are the site of protein synthesis
• Transfer RNA (tRNA) brings the amino acidsto the ribosomes during protein synthesis

Synthesis of mRNA

• Transcription of DNA into RNA is accomplished by RNA polymerase
• RNA polymerase binds to the DNA molecule and copies a template strand
• The enzyme binds and moves in the 3'to 5' direction along the DNA strand
• Synthesizes the mRNA in the opposite polarity; 5' to 3' direction

Transcription in Procaryotes

• mRNA synthesis occurs over discrete sequences of nucleotides (genes)
• Initiation of transcription occurs at the promoter
• Promoters is a specific nucleotide sequences which is recognized by the active site of the RNA polymerase
• Reading 5' to 3' along the complementary strand is a TTGACA followed 17 (or so) nucleotides in the 3' direction by a TATATT
• Promoter sequencesare called consensus sequences because they are found in front of all genes
• The TATA sequences seems to be most important

Termination of Transcription in Procaryotes

• Once the Rna polymerase binds it will initiate transcription at a start site (usually a CAT located about 7 bases toward the 3'end of the complimentary strand
• Transcription will continue until a termination sequence is encountered
• Sequence of bases that allow complimentary bases to form a "hairpin" loop
• Followed by a run of 4 to 8 A's

Procaryotic Termination

• Formation of the haipin loop allow the DNA duplex to reform, putting stress on the polymerase.
• The run of adenine residues form only 2 complimentary hydrogen bonds with newly polymerized uracil residues and causes the polymerase to pause
• Weak binding the pause disengage the polymerase

Transcription in Eucaryotes

• Eucaryotic transcription is more complicated
• Eucarytic mRNA is tagged at both ends
• 7-methylguanosine is added to the 5' end
• A tail of 100-200 adenines is added to the 3' end
• The original copy of the template strand is referred to as the primary transcript
• Dramatic differences exist between the primary transcript and the functional mRNA

Modification to the Primary Transcript

• Primary transcripts contain contain large region of nucleotides that are nonfunctional in the mRNA
• These nonfunctional regions are called introns and must be processed or removed from the transcript
• The regions that survive the processing are called exons
• Exons encode the primary sequence of proteins

Splicing Process

• Intron are removed early but are important in the functioning of the message and their transport to the cytoplasm
• The beginings and ends of intronsare marked
• The boundary between introns and exons are recognized by small nuclear ribonucleoproteins (snRNPs)
• Four types of participate in the excision of introns
• Catalyze the breakage and removal of the intron
• Other snRNP link the exons together

This page is maintained by James C. Pushnik: jpushnik@ecst.csuchico.edu

Last modified 11/11/96